1. Field of the Invention
The present invention relates to a device for protection against the parasitic effects caused by negative impulses of power supply voltages in monolithic integrated circuits including an inductive load, a power device for driving the inductive load and a control device for the power device.
2. Prior Art and Other Considerations
Circuit assemblies of this type are known, in particular for motor car use, which can be accomplished alternatively on two or one slabs.
In the case in which the power device and the control device are integrated on two different slabs a classic solution for protecting the power device from negative impulses of the power supply voltage consists in using an external resistance and a diode connected to the power supply and in parallel to the control device, respectively. The diode operates so that the negative voltage across the control device is not higher than its direct conduction voltage; the external resistance is used to limit the maximum current which can flow through the diode. If the diode and the external resistance are of the correct size, the solution is sufficient to prevent drawbacks such as excessive conduction, and on occasion the burning out of components internal to the integrated circuit.
But if it is desired to integrate the power device monolithically with the control device, the described protection is not sufficient.
In fact, with this technological solution, there is the formation of a parasite transistor having the collector connected to the collector of the power device and the emitter connected to the power supply.
When a negative impulse arrives on the power supply, the power device (say, of the darlington type) is quenched with a negative base tension, thus very quickly.
If the negative impulse arrives during the interval in which the power device is fired, that is when the coil on the collector of the power device has energy stored, the collector voltage of the same rises suddenly to a clamp voltage which, for example, can be set by a high-voltage zener diode whose cathode is connected to the collector of the power device.
This condition translates into an inverse secondary breakdown stress for the power device, whose quenching speed is much greater than in the case of a normal operation, when the darlington base can only be quenched to ground, while in the case examined it is quenched to a negative voltage. There is simultaneously present a direct secondary breakdown stress for the parasite transistor, which must withstand the high voltage of the power device and the current due to the fact that its base-emitter voltage is taken positive.
In order to avert such drawbacks, as shown in the simultaneous Pat. application No. 21150 A/89 filed on 11 Jul. 1989 in the name of the same applicant, the introduction is provided, inside the control device, of a voltage limiting circuit essentially constituted by a first transistor sensitive to negative impulses on the power supply and by a second transistor controlled by said first transistor and suitable for controlling the re-firing of the power device.
This solution, which in substance provides for the limitation of the collector voltage of the power device, is valid for solving the problem of negative overvoltages, and thus for preventing the destruction of the parasite transistor connected between the collector of the power device and the power supply; but due to the effect of the integrated structure, the formation is determined, between the base of said second control transistor and the power supply, of a further parasite transistor which adsorbs the current which should re-fire the power device and thus renders ineffective the solution described above.
The object of the present invention is to eliminate the effect of the further parasite transistor.